Abstract Despite significant progress in Alzheimer's disease (AD) in general and tau pathobiology in particular, there is still no effective treatment or prevention for AD or any other tauopathy. To develop such therapy, detailed knowledge of the structural biology and structure?activity relationship of tau is needed, including the way sequence alterations and post-translational modifications affect tau self-assembly into toxic oligomers and aggregates, and how these parameters impact tau seeding and toxicity. Here, we propose a systematic, detailed study of these aspects of tau pathology taking advantage of recent developments in mass- spectrometric, biochemical, and cell biology methods. We will study the effect of primary-structure alterations and post-translational modifications on tau oligomerization and aggregation in vitro and compare recombinant and in-vivo generated tau. We will then examine how all of these factors affect tau seeding using a recently developed highly sensitive biosensor cell line. To advance therapy development, we will also test the effect of assembly modulators on tau self-assembly and seeding. A unique aspect of the project is the combination of expertise of the PI and Co-I groups, which will allow obtaining insight into the structure?activity relationship of tau on multiple levels and answering currently pending questions about the complex processes governing this crucial aspect of AD.